Influence of the estrous cycle and estradiol on the behavioral effects of amphetamine and apomorphine in rats

Abstract

This experiment was designed to investigate the influence of hormonal status of the rat on the effects of two doses of an indirect-acting dopamine agonist (amphetamine 0.25 and 1.0 mg/kg, IP) and a direct-acting dopamine agonist (apomorphine 62.5 and 250 μg/kg, SC) on the acquisition of conditioning avoidance responses (CARs) and the performance of some spontaneous behaviors. Active conditioned avoidance was improved by amphetamine in all the groups except in females at diestrus; apomorphine improved this response only in females at estrus and in ovariectomized rats after estradiol replacement, but the avoidance response was deteriorated in males and females at diestrus and after ovariectomy without estradiol replacement. Both dopaminergic drugs had contrasting effects on motor activity, number of rearings, and number of head shakes according to the hormonal status of the rat. Only the time spent in grooming behavior decreased after the treatment with both dopamine agonists in all of the five groups studied. These results provided behavioral evidence for the hypothesis that dopaminergic activity in the CNS is affected distinctively by modifications in the sexual hormone status (gender, estrous cycle, ovariectomy, and estradiol replacement). Relationships between ovarian hormones and dopaminergic system are discussed.